Over the past few years, several "perpendicular recording media" so called because the recording medium is magnetized perpendicular to the surface of its magnetiazble thin film, have been proposed for increasing magnetic recording density. Japanese Patent Publication No. 58-91 discloses one perpendicular recording medium comprising a polyimide substrate and a two-layer magnetic thin film thereon, one layer on the substrate being a low-coercive force layer of molybdenum/iron/nickel and the other layer thereon being a magnetic recording layer of cobalt/chromium. Magnetic thin films of this two-layer structure type provide a number of advantages. Since the magnetic circuit on the back surface of the perpendicularly magnetizable cobalt/chromium film is partially closed by the high permeability, low coercive force magnetic layer, magnetic deterioration is minimized and residual magnetization is enhanced.
Magnetic thin films having the overall desired properties can be obtained by making the magnetic thin film a multilayer structure consisting of layers with different properties.
It has hitherto been the practice to form a magnetic thin film having such a two-layer structure by heating the non-magnetic substrate to temperatures above room temperature (250.degree. C. in examples described in Japanese Patent Publication No. 58-91), forming a low coercive force, high permeability magnetic layer thereon by sputtering, and further forming a magnetic recording layer of cobalt/chromium thereon by sputtering. Referring to FIG. 1, a low coercive force, high permeability magnetic layer 1 formed by this process has an average grain diameter B which is equal to or more than the average grain diameter A of a cobalt/chromium layer 2. More perticularly, the surface of the underlying high permeability magnetic layer is microscopically so irregular as to affect the overlying cobalt/chromium layer 2 being formed, resulting in the disordered orientation of columnar cobalt/chromium grains with deteriorated surface planeness.
Further, various crystal faces including (110), (111) and (100) are exposed on the surface of the low coercive force, high permeability magnetic layer. If a magnetic recording layer destined for perpendicular magnetization is formed on this high permeability magnetic layer, the random crystal faces on the surface of the underlying magnetic layer will affect the overlying magnetic recording layer being formed particularly at the initial stage of the process. When a permalloy film is used for the low coercive force, high permeability magnetic layer and a cobalt/chromium film is used for the magnetic recording layer destined for perpendicular magnetization, there is a likelihood of cobalt/chromium grains growing with their (001) face in conformity with the (111) face of the permalloy grains. That is, the cobalt/chromium grains tend to grow in the directions of arrows shown in FIG. 2. As a result, cobalt/chromium grains are randomly oriented to form a cobalt/chromium film having an irregular surface as shown in FIG. 1.